1. Field of the Invention
The present invention relates to an image heating apparatus suitable for a heat-fixing apparatus mounted to an image forming apparatus such as an electrophotographic copier and an electrophotographic printer, and a pressure roller used in the image heating apparatus.
2. Description of the Related Art
Among heat-fixing apparatuses mounted to electrophotographic printers or electrophotographic copiers, a thermo roller-type heat-fixing apparatus is known which includes a halogen heater, a fixing roller heated by the halogen heater, and a pressure roller forming a nip part by being brought into contact with the fixing roller. Additionally, a film heating-type heat-fixing apparatus is known which includes a heater having a heating resistor formed on a ceramic substrate, a fixing film sliding on the heater, and a pressure roller forming a nip part together with the heater with the fixing film interposed therebetween. Both the thermo roller-type heat-fixing apparatus and the film heating-type heat-fixing apparatus are configured to heat-fix a toner image onto a recording material by pinching and conveying the recording material bearing unfixed toner image thereon via the nip part.
In the printer mounted with the thermo roller-type heat-fixing apparatus, when small-size recording materials are continuously printed at the same print interval as in a case of printing large-size recording materials, an excessive temperature rise occurs in a non-paper passing area of the fixing roller, the non-paper passing area indicating an area where the recording material does not pass. Additionally, in the printer mounted with the film heating-type heat-fixing apparatus, when small-size recording materials are continuously printed at the same print interval as in a case of printing large-size recording materials, an excessive temperature rise occurs in a non-paper passing area of the heater. When the excessive temperature rise occurs in the non-paper passing area of the fixing roller or the heater, respective parts included in the heat-fixing apparatus may be damaged. Additionally, when the large-size recording material is printed in a state where the excessive temperature rise occurs in the non-paper passing area, the toner on the recording material at the non-paper passing area is melted too much, thereby causing a high-temperature offset.
Particularly, in case of the film heating-type heat-fixing apparatus, since the thermal capacity of the heater is smaller than that of the thermo roller-type heat-fixing apparatus, a large temperature rise occurs in the heater at the non-paper passing area. For this reason, the durability of the pressure roller deteriorates or a high-temperature offset occurs easily. Additionally, the film is rotationally driven in an unstable state, or the film is easily twisted to be wrinkled.
Additionally, as the process speed of the printer becomes faster, the temperature rise occurs easily at the non-paper passing area. This is because the time necessary for the recording material to pass through the nip part becomes short in accordance with an increase in the speed of the printer, and thus the fixing temperature necessary for heat-fixing the toner image onto the recording material should be increased. Also, this is because the paper-interval time during the continuous printing process is reduced in accordance with an increase in the speed of the printer, the paper-interval time indicating a time when the recording material is not interposed in the nip part, and thus the temperature distribution cannot be controlled to be uniform during the paper-interval time.
As one of techniques of reducing the excessive temperature rise at the non-paper passing area, a technique is known which sets the thermal conductivity of the pressure roller to a large value. In terms of this technique in which the thermal conductivity of the elastic layer included in the pressure roller is increased, it is possible to reduce the temperature when the temperature rise occurs at the non-paper passing area, that is, a difference between a high temperature and a low temperature in a longitudinal direction of the pressure roller.
Japanese Patent Application Laid-Open No. H11-116806, Japanese Patent Application Laid-Open No. H11-158377, and Japanese Patent Application Laid-Open No. 2003-208052 disclose a technique in which a high thermal conductive filler, such as alumina, zinc oxide, or silicon carbide, is added to base rubber in order to increase the thermal conductivity of the elastic layer of the pressure roller and the fixing roller.
Japanese Patent Application Laid-Open No. 2002-268423 discloses a technique in which a carbon fiber is contained in an elastic layer in order to increase thermal conductivity of a rotary body (fixing belt instead of pressure roller) having the elastic layer.
Japanese Patent Application Laid-Open No. 2000-39789 discloses a technique in which an anisotropic filler, such as graphite, is contained in an elastomer layer in order to increase the thermal conductivity in a thickness direction of a roller.
Japanese Patent Application Laid-Open No. 2002-351243 discloses a technique in which a fabric layer using pitch-based d carbon fiber is provided in an elastic layer of a pressure roller. The pressure roller includes the elastic layer and a high thermal conductive layer having excellent thermal conductivity. However, since it is a fabric or a structure corresponding thereto, the hardness of the high thermal conductive rubber composite layer increases. Therefore, in a case where the hardness of the pressure roller decreases as a whole, a countermeasure is supposed in which foamed sponge rubber is used as a lower elastic layer. However, since the elastic layer is configured as the foamed sponge rubber, the durability of the pressure roller is not good. For this reason, the pressure roller is suitable for a pressure member mounted to a low-speed image forming apparatus.
Japanese Patent Application Laid-Open No. 2005-273771 corresponding to U.S. Pat. No. 7,321,746 discloses a technique in which pitch-based d carbon fiber is dispersed in an elastic layer of a pressure roller.
Even when the filler, such as alumina, zinc oxide, silicon carbide, carbon fiber, or graphite described in the above-described Patent Documents, is added in order to increase thermal conductivity, it is not possible to obtain the desired thermal conductivity if a small amount of filler is added. On the other hand, if a large amount of filler is added, the hardness of the pressure roller tends to increase too much, and thus it is difficult to ensure the fixing nip width.
In Japanese Patent Application Laid-Open No. 2005-273771 corresponding to U.S. Pat. No. 7,321,746, the thermal conductivity in a longitudinal direction of the pressure roller is excellent, and the hardness of the roller can be appropriately set. However, since the heat transmission from the elastic layer to the metal core is very good, a problem arises in that the surface temperature of the roller decreases too much. In a case where the surface temperature of the pressure roller is too low, vapor produced when the recording material passes through the heating nip part is condensed in the surface of the pressure roller, thereby causing a problem in that the recording material is conveyed in an unstable state.
Therefore, it may be supposed that the elastic layer is formed into a two-layer structure and the outer elastic layer is configured as a high thermal conductive layer.
However, even in case of the two-layer elastic layer, the center nip width of the nip part is narrower than the end-portion nip width thereof in a longitudinal direction of the pressure roller. Accordingly, upon heat-fixing the unfixed toner image onto the recording material, fixability at the center of the nip part may be not sufficient, or fixability at the end portion of the nip part may be excessive.